This invention relates in general to flight simulation, and more particularly, to an improved illustration of terrain for use in a flight simulator of the flying spot scanner variety and a process for illustrating terrain features such that the resulting illustration may be used for flight simulation.
Flight simulators, which are used extensively in training and qualifying pilots for commercial as well as military aircraft, require a display that represents the view from the cockpit of an aircraft in flight. This display appears in front of the cockpit and must change or move to simulate forward motion of an aircraft. It must further change in response to manipulation of controls that affect aircraft direction, altitude, pitch angle, and roll angle. Some displays take the form of imagery which is projected onto a spherical screen or dome that completely surrounds the simulated cockpit. Other displays take the form of imagery that appears on a television monitor located in front of the simulated cockpit.
The images that comprise the display may be derived from several sources, one of the least complicated being a television system that employs a flying spot scanner. In this system a CRT raster is projected upon and moves over a transparency that is a simple aerial photograph or perhaps a mosaic composed of a multitude of aerial photographs. The position of the projected raster on the transparency is controlled by a computer which responds to commands given by a pilot in the simulated cockpit. That portion of the transparency through which the raster projects is reproduced as an image on the screen or monitor, with appropriate corrections made electronically for pitch angle, roll angle, yaw angle, and altitude. The aircraft position coordinates, that is the location of the projected raster on the transparency, may be controlled electronically if the transparency is small enough. Otherwise, they are controlled by "X" and "Y" servo motors which move the transparency.
The basic flying spot scanner system possesses several major disadvantages. First, it occupies considerable space because the transparency is either in the form of a roll, usually about 6 inches wide and several feet long, or else in the form of a square plate, some measuring about 4.times.4 feet. Secondly, images are rarely orthonormal, because the aerial photographs from which they are derived are not orthonormal throughout. In this regard, every portion of the scene should be presented as if viewed from directly above, but aerial photographs covering large areas do not conform with this objective. Third, the flying spot scanner does not afford adequate contrast, at least when a projected image is employed. In this regard, aerial photographs contain many shades of gray or other regions of low contrast. These areas exhibit greater contrast at simulated high altitude flight than at simulated low altitude flight, which is opposite of what happens in actual flight. In other words, as an aircraft descends in actual flight, the scene observed by the pilot will increase in contrast, and not decrease as will occur with a flying spot scanner system. The problem is particularly acute with systems that project images onto dome-shaped screens, for the screens must have matte surfaces, that is, surfaces which are diffuse and not too highly reflective, to avoid nonuniform and excessive reflections within the domes themselves. Such reflections will of course reduce the contrast significantly. In this same vein, the image quality derived from conventional flying spot scanner systems is not very good. The same holds true with regard to image control, particularly where the transparency is a mosaic, for it is difficult to conceal the boundaries of the individual photographs that comprise the mosaic. Indeed, while aerial photographs are readily available from the Federal government, selection of the appropriate photographs and piecing them together into a workable mosaic is a difficult and burdensome task.